1. Field of the Invention
This invention relates to the provision of a new class of shortstop agent for use in the polymerization of vinyl monomers. The new class of shortstop agents are compounds which form stable nitroxyl free radicals. Advantages of the use of this new class of materials over those currently used include less volatility, less probability of forming nitrosamine precursors, and superior performance in terminating free radicals.
2. Introduction
In free-radical emulsion polymerization, the properties of the polymer formed are dependent upon monomer conversion. The best properties of the polymer often occur at 55 to 75% conversion. The ability to "shortstop" the polymerization is often critical to the production of polymers having the proper physical characteristics. Termination of polymerization at a particular time is especially critical in the production of synthetic rubber. By the term "shortstop" as used herein is meant a process for ending polymer chain propagation at a specific point in time so as to obtain desirable polymer properties including molecular weight distribution.
Free-radical polymerization consists of three distinct mechanisms: 1) initiation of alkyl radical by catalyst, 2) propagation of polymer chain by addition of monomer to growing alkyl radical, and 3) termination of growing alkyl radical by reaction with another radical or with another substance that forms a product which either retards or stops further reactions with monomers. It is recognized that a reaction can be stopped by destruction of the initiator, by prevention of propagation, or by termination of the growing alkyl radical. Usually a shortstop is selected that works on all three mechanisms. Shortstop agents and the reason for utilizing shortstop agents in the production of synthetic rubber latex are disclosed in "Polymerization-Stopping Agents, G. J. Antlfinger and C. H. Lufter, Industrial and Engineering Chemistry, Volume 45, No. 1, pages 182-185 January, 1953. Usually some degree of polymerization or crosslinking continues after shortstop agents are added.
Two of the most successful current commercial shortstop agents are diethyl-hydroxyl amine and isopropylhydroxyl amine. Another example of a commercial shortstop agent is a combination of sodium dimethyldithiocarbamate (DTC) and diethylhydroxyl amine (DEHA). These work in conjunction to very effectively stop the polymerization. However, this combination has recently come under scrutiny because these components are nitrosamine precursors, (Crosslinking systems that avoid nitrosamines, Ehrend, Helfried, Rubber and Plastics News, Oct. 30, 1989, page 17-18). U.S. Pat. No. 5,384,372 to Lattime recognizes the nitrosamine generation problem and proposes the use of isopropylhydroxylamine and salts thereof to avoid nitrosamine formation. Likewise EP Application 656,371A2 to Maestri also deals with this potential problem by proposing the use of isopropylhydroxylamine salts in combination with a polysulfide. Recent patents show efforts to develop shortstop materials which avoid or attempt to minimize the formation of nitrosamine materials. These patents typically disclose the use of materials which do not contain or degrade to secondary amines. DE 88-3836777 A1 Hoerpel et al. discloses the use of hydroxydithiobenzoic acid and its water soluble salts as free radical deactivators. Generally a sulfur compound is needed in conjunction with a hydroxylamine to both degrade the catalyst and terminate alkyl radicals. When either material is used alone, we believe that detrimental changes in the polymer occur as measured by changing Mooney viscosity.
The stable nitroxyl free radical (SNFR) is known to quickly combine with alkyl free-radicals to terminate polymerization. The subject compounds are known to prevent polymerization from propagating during the manufacture, purification, and storage of vinyl monomers such as butadiene, styrene, acrylates, acrylonitrile and other vinyl monomers. The use of stable nitroxyl radicals in this fashion are disclosed in U.S. Pat. No. 4,670,131 and U.S. Pat. No. 5,254,760, the disclosures of which are both hereinafter incorporated by reference into this specification. While SNFRs have been used in the applications above, they have not been utilized to the knowledge of applicants as shortstopping agents for emulsion polymerization of vinyl or diene monomers, and particularly in the preparation of synthetic rubber. While SNFRs are used at very low concentrations to inhibit polymerization of vinyl monomers during manufacture, purification, storage and transport where polymerization is due to incidental free radicals from heat or oxygen, they have not been utilized as shortstopping agents in polymerizations that are initiated by free-radical catalysts for the express purpose of producing polymer. This is especially true when the free-radical catalysts are added in such quantity to cause polymerization to proceed quickly. The SNFRs are known not to react readily with peroxy initiators or their free-radicals. Also, because of the nitroxyl functionality, they may have been cursorily dismissed as nitrosamine precursors. For these reasons SNFRs have not been used as shortstop agents.
Surprisingly, because SNFRs react so rapidly with alkyl radicals, they have proven to be very effective as shortstop agents in vinyl and diene free-radical polymerizations, and particularly the free-radical polymerization to produce various latex rubbers. Even though a free-radical catalyst used to manufacture latex rubber may still be generating free-radicals that initiate alkyl radicals, the rapid terminations of alkyl radicals by SNFRs do not allow for changes in conversion or molecular weight.
Examples of SNFRs are given in Winter and Ferrell both previously incorporated by reference into this specification. A particularly preferred SNFR is 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl. An example of a compound that would form a SNFR in situ is 1,4-dihydroxy-2,2,6,6-tetramethylpiperidinium formate. It would react with peroxy catalyst to "kill" the catalyst and produce a SNFR. Diethylhydroxyl amine (DEHA) and isopropylhydroxyl amine (IPHA) do not form SNFRs. Though compounds which kill the catalyst may not be needed when SNFRs are used, an effective amount of SNFR employed as shortstopping agents may be used in combination with DEHA, IPHA, or sulfur compounds such as sodium polysulfide or hydroxydithiobenzoic acid when such materials are utilized.
An additional improvement, over DEHA/DTC combination is believed to be the reduced level of volatile nitrosamine precursors. This is because of the difficulty of forming nitrosamines with SNFRs. If nitrosamines form, it is believed that the relatively high molecular weight of SNFRs would make resultant nitrosamines non-volatile.
Another benefit of the SNFR is its higher molecular weight and subsequent reduced volatility resulting in reduced removal of the shortstopping agent from a vinyl or diene polymerization, and particularly a polymerization in which the desired product is a synthetic rubber during monomer stripping. Still another advantage is that the SNFR, being relatively non-volatile is more likely to stay with the polymer during continued processing such as drying. Continued shortstopping of rubber polymerization during post polymerization steps will result in less change in the polymerization and an improved finished rubber product. The use of SNFRs would accordingly limit changes to the Mooney viscosity (molecular weight) by reducing gel. Gel is defined as areas of insoluble polymer caused by crosslinking of rubber molecules. The advantage for the use of SNFRs would be quicker and more complete termination of polymerization when used as a shortstopping agent.